Plasmids are stable extra-chromosomal genetic elements in bacteria that are most commonly comprised of circular DNA (often a few kilobase-pairs [kb] in size) and that are capable of autonomous replication within bacterial cells. With the advent of recombinant DNA technology, plasmids gained use in a variety of processes as vehicles or vectors for the introduction and maintenance of non-native DNA in bacterial host cells. More recently, plasmids have become important as products as well, in the form of DNA vaccines in medical and veterinary practice. In most such applications, the bacterial host is Escherichia coli. 
An important feature in relation to plasmids is their copy-number regulation within bacterial cells. Under ordinary growth conditions, each plasmid is maintained at a characteristic copy number in the cell by the operation of feedback regulatory loops that control replication of the plasmid DNA. One class of plasmids which is extensively employed in biotechnology are the “ColE1-like” plasmids which include the naturally occurring plasmids ColE1, p15A, RSF1030, CloDF13, and pMB1 as well as the cloning vectors pBR322, pBR329, pACYC184, pACYC177 and the pUC and pBluescript series of plasmids. In the replication of the ColE1-like plasmids in E. coli, the feedback regulatory loops involve a plasmid-specific RNA species (RNA-II) that activates replication, and a second plasmid-specific RNA species (RNA-I) and a protein (Rop) that act to inhibit replication [Polisky (1988) Cell 55:929–932].
Alterations in copy numbers of plasmids—both upward and downward—have been sought to be achieved by manipulation of the concerned feedback regulatory loops in the prior art, as briefly described below. Manipulations to increase the plasmid copy number are in general desirable when one's aim is (i) to increase the yields of plasmid DNA from cultures, for example, in routine recombinant DNA experiments or in production of DNA vaccines; or (ii) to increase the expression of product(s) encoded by plasmid-borne gene(s). Increase in plasmid copy numbers have been achieved either constitutively by mutations inactivating the feedback regulatory circuits (present, for example, in the commonly used ColE1-derived pUC series of plasmid vectors with very high copy numbers, such as pUC18 or pUC19) or by inducible processes that have been referred to as runaway-plasmid-replication systems [Lin-Chao et al. (1992) Mol. Microbiol. 6:3385–3393; Nordstrom and Uhlin (1992) Bio/Technology 10:661–666].
Manipulations to decrease the plasmid copy number become valuable in situations where the gene(s) borne on the plasmid confer a dosage-dependent growth disadvantage to the host bacterial cells. Mutation in pcnB, an E. coli chromosomal gene, has been shown to reduce the copy number of both ColE1-derived and p15A-derived plasmids, most probably through stabilization of the RNA-I transcripts involved in the negative control of plasmid replication [Lopilato et al. (1986) Mol. Gen. Genet. 205:285–290].